In a conventional vehicle, an internal combustion engine generates rotary power by burning fuel, and the power is transmitted to the driving wheels through a mechanical transmission. The engine is the only power supply of the vehicle and has to provide all the torque that the vehicle needs. When cruising on a flat road, the vehicle only needs relatively small power or torque to maintain its speed, but it takes much more power or torque to accelerate or go uphill. The engine has to able to deliver the peak torque required for acceleration and uphill, so a very powerful engine usually is required. Although the peak power is needed only a small fraction of working time, the engine has to overcome friction and keep running. The bigger the engine, the stronger the friction, and the more energy is required to maintain the engine running.
When the brake is applied, the kinetic energy of the vehicle is converted into heat, losing a large amount of energy. Also the conventional vehicle keeps burning fuel when it stops at traffic light.
In order to improve fuel efficiency and reduce emission, many hybrid electric vehicles are proposed: one or more electrical motors are used to assist the engine when high torque is needed; the motor(s) also can re-generate electric power while applying brake torque on the wheels; a power battery stores electric energy when it is available and provides the energy to the motor(s) when it is needed.
A so called “full hybrid vehicle” has the abilities of: shutting down engine when the vehicle stops, running solely on electrical power at low and medium speed, starting engine when the vehicle reaches a certain speed, regenerating electricity while braking; and assisting the engine with electric power when needed.
A hybrid electric drive system is proposed in U.S. Pat. No. 5,934,395. It is a full hybrid vehicle. The vehicle has an engine, two electric motors, an inverter for each motor, a battery, and a distributing mechanism.
The distributing mechanism splits the engine power/torque between one of the motors and the drive shaft. A certain fraction of the engine power/torque goes to the motor, and the rest goes to drive shaft. In other words, not all the engine torque can go to the drive shaft directly, even though the engine torque is crucially needed. This causes a slow performance or claims a high capacity of motor drives.
The purpose of this invention is to provide a full hybrid electric drive system which can fully use of the engine torque to drive the vehicle.